Bauxite is the basic raw material for almost all manufactured alumina compounds. In the course of production of aluminum compounds, most bauxite is refined to aluminum hydroxide by the Bayer process. The Bayer process involves hot leaching of bauxite with NaOH solution in pressure vessels to obtain supersaturated sodium aluminate solutions from which Al(OH)3 is precipitated by seeding. Bayer process refineries share six common process stages: bauxite mining; raw material preparation; bauxite digestion; separation, washing, disposal of insoluble bauxite residue; aluminum hydroxide (trihydrate) precipitation; and calcinations to anhydrous alumina. The process of separating bauxite residue solids from the supersaturated green liquor near its boiling point is known as “clarification”.
In the clarification stage, the coarser solid particles are generally removed with a “sand trap” cyclone. To separate the finer solid particles from the liquor, the slurry is normally fed to the center well of a mud settler where it is treated with a flocculant composition that may be based on a variety of flocculating agents including starch, flour, polyacrylate salt polymer, acrylate salt/acrylamide copolymer, and/or water-soluble polymers containing pendant hydroxamic acid or salt groups. As the mud settles, clarified sodium aluminate solution, referred to as green liquor, overflows a weir at the top of the mud settling tank and is passed to the subsequent process steps. The sodium aluminate solution is generally cooled to enhance supersaturation and then seeded, e.g. with fine gibbsite seed from previous cycles to initiate precipitation of the desired end product Al(OH)3.
The settled solids from the flocculation procedure, known as red mud, are withdrawn from the bottom of the mud settler and passed through a countercurrent washing circuit for recovery of sodium aluminate and soda. Aluminate liquor overflowing the settler may still contain significant amounts of suspended solids. This liquor is generally further clarified by filtration to give a filtrate that contains a very low level of suspended solids. Depending on the level of silicates in the bauxite, the red mud and/or aluminate liquor may contain sodium aluminosilicates. Dissolved sodium aluminosilicates may precipitate to form scale. Insoluble sodium aluminosilicates, also known as desilication product (DSP), may remain suspended in the red mud and/or aluminate liquor.
Alumina in relatively pure form is precipitated from the filtrate as alumina trihydrate crystals. The remaining liquid phase is returned to the initial digestion step and, after being reconstituted with additional caustic, is employed as a digestant of additional ore.
The suspended solids are preferably separated at a relatively fast rate if the overall Bayer process is to be efficient. Efficient removal of suspended solids from Bayer process streams has been a major challenge for many years. Among the methods of speeding up separation of suspended solids from process streams as well as providing a cleaner separation of the constituents are those disclosed in U.S. Pat. No. 3,390,959, which employs polyacrylates as flocculants, and U.S. Pat. No. 3,681,012, which uses combinations of polyacrylates and starch in Bayer alumina recovery circuits. U.S. Pat. No. 4,083,925 discloses the use of polyacrylamide within the mud settler. U.S. Pat. No. 4,678,585 teaches that different stages in the Bayer alumina recovery circuit are advantageously treated with different flocculant compositions. U.S. Pat. No. 4,767,540 describes a process for removing suspended solids from Bayer alumina process streams by contacting and mixing a Bayer process stream with hydroxamated polymers. The hydroxamated polymers may be employed with anionic polyacrylate. U.S. Pat. Nos. 5,516,435 and 5,539,046 use blends of hydroxamated polymer emulsions with polyacrylate emulsions to remove suspended solids from Bayer alumina process streams. Other polymers disclosed for the treatment of red mud in the Bayer process include phosphonic acid-containing polymers (U.S. Pat. No. 5,534,235), water continuous methyl acrylate emulsion polymers (U.S. Pat. No. 6,036,869), and salicylic acid containing polymers (U.S. Pat. No. 6,527,959).
Silicon-containing polymers have been disclosed for water clarification. For instance, U.S. Pat. No. 3,779,912 uses silicon-containing aminomethylphosphonates to flocculate suspended solids in water. Copolymers of diallydimethylammonium halide and a vinyltrialkoxysilane are disclosed as a coagulant used in demulsification of oily waste waters (U.S. Pat. No. 5,560,832), dewatering of mineral slurries (U.S. Pat. No. 5,597,475), and clarification of waste waters (U.S. Pat. No. 5,679,261). U.S. Pat. No. 6,605,674 discloses the use of vinyltrialkoxysilanes as cross-linking agents to modify structure of nonionic, cationic and anionic water-soluble polymers and the use of the structurally-modified polymers as flocculating agents. None of the above-mentioned silicon-containing polymer patents relate to the treatment of suspended solids from the Bayer process streams.
The use of silicon-containing polymers to control aluminosilicate scale has been disclosed, see U.S. Pat. No. 6,814,873 and U.S. Pat. Pub. Nos. 2004/0162406 A1, 2005/0010008 A2, and 2005/0274926 A2. These publications describe methods for using the silicon-containing polymers to inhibit dissolved aluminosilicates (such as sodium aluminosilicate) from depositing on surfaces to form scale, but not for flocculating DSP.
It has been now discovered that greatly improved flocculation of suspended solids, especially DSP, from Bayer process streams may be obtained by adding and efficiently mixing a silicon-containing polymer into the Bayer process stream alone or subsequent to, followed by or in association with a conventional flocculant. This treatment is particularly effective in treating bauxite residue solids containing high silicates and sodium aluminosilicates when compared with state-of-the art processes, as exemplified by the patents mentioned above. Such reductions in suspended solids can significantly reduce the need for filtration. Since the suspended solids may contain undesirable impurities, the reductions in suspended solids achieved by practice of the present invention may also result in improved purity of the resultant alumina product.